Raman scattering from the effective soft mode in lead germanate crystal

The Raman opalescence of an effective soft mode in crystalline lead germanate is investigated. Recording of Raman scattering intensity was in the x (zz)y geometry corresponding to the soft mode, responsible for ferroelectric phase transition point at 451 K. The Raman spectra were measured at fixed temperatures T (I(omega ', T); T = const) and also at fixed wavenumbers omega ' of Raman spectrometer (I(omega ', T); omega ' = omega(0) - omega = const) with slow temperature changing. The strong increase of Raman scattering intensity (Raman opalescence) has been observed at temperatures of phase transition region. The appearance of central peak scattering is attributed to the strong interaction of the effective soft mode with a low-wavenumber nonfundamental oscillator.